1. Field of the Invention
The present invention relates to a ceramic capacitor and a manufacturing method thereof and in particular to a ceramic capacitor for high voltage applications and a manufacturing method thereof.
2. Prior Art
A conventional ceramic capacitor 1 is known, for example, as shown in FIG. 4, which is made by forming an electrode 3 on both principal surfaces of a dielectric ceramic element 2 based on barium titanate or strontium titanate. This electrode 3 is formed on both principal surface by applying, for example, silver paste on both principal surfaces of the dielectric ceramic element 2 and subsequently baking it.
FIG. 5A is an illustrative view showing the cruving of the equipotential surfaces in the vicinity of the edge portion of the ceramic capacitor. As seen from FIG. 5A, the potential inclination is increased in the vicinity of the edge portion of the electrode and the electric lines of force are concentrated. Hence, when a high voltage is appliced, dielectric breakdown is likely to take place between the edge portions of both electrodes.
In order to prevent such concentration of electric lines of force in the vicinity of the electrode, it is conceivable of form both electrodes to entirely both principal surfaces. In this case, the state of the equipotential surfaces is shown in FIG. 5B, wherein the potential inclination in the vicinity of the edge portion of the electrode is definitely less than that shown in FIG. 5A. Hence, dielectric breakdown between the edge portions of both electrodes of the dielectric ceramic element in the edge portion is less likely to take place, and the dielectric breakdown voltage of the ceramic capactior can be increased. In our experiment, too, as shown in FIG. 6, it was confirmed that the shorter the distance between the end (edge) of the dielectric ceramic element and the end (edge) of the electrode, the larger the dielectric breakdown voltate.
When, however, the electrodes are formed on both entire main surfaces of the dielectric ceramic element, it is difficult to maintain the ideal potential inclination. This problem may result in lowering of the dielectric breakdown voltage, if for example the edge portion of the ceramic capacitor should be chipped off or cracks should be formed therein. Also, when the electrode is formed by coating both main surfaces with a silver paste and subsequent baking, the silver paste tends to flow and thereby stain the sides of the dielectric ceramic element. In this case, an additional step of removing the silver paste from the sides of the dielectric ceramic element is required, and the manufacturing cost of the ceramic capacitor is increased.
For these reasons, in practice, the electrode is not formed all over both main surfaces of the dielectric ceramic element but is so formed that there is some distance between the edge of dielectric ceramic element and that of the electrode. Hence, the actual dielectric breakdown voltage of the ceramic capacitor is less than the maximum dielectric breakdown voltage shown in FIG. 6.